Measurement of blood flow in the heart and blood vessels by using the Doppler effect is well known. While amplitude of the reflected waves is employed to produce black and white images of the tissues, the frequency shift of the reflected waves may be used to measure velocity of reflecting scatterers in tissue or blood. Color flow images are produced by superimposing a color image of the velocity of moving material, such as blood, over the black and white anatomical image. The measured velocity of flow at each pixel determines its color.
A major difficulty in making Doppler effect measurements of ultrasonic waves reflected from blood is that the received echo signal typically contains a large component produced by stationary or slowly moving tissues, whereas blood reflects ultrasound very weakly. The stationary tissues do not produce a frequency shift in the reflected waves and these components can easily be filtered out without affecting the flow measurement. However, the reflections produced by the moving tissue due to cardiac or respiratory motion are frequency shifted and may completely overwhelm signals from slowly flowing blood.
In standard color flow processing, a high pass filter known as a wall filter is applied to the data before a color flow estimate is made. This filter removes signal components produced by tissue surrounding the blood flow of interest. If these signal components are not removed, the resulting velocity estimate will be a combination of the velocities of the blood flow and the surrounding tissue. The backscatter component from tissue is many times larger than that from blood, so the velocity estimate will most likely be more representative of the tissue, rather than the blood flow. In order to measure the flow velocity, the tissue signal must be filtered out.
Most commonly, color flow processors accept the large signal returning from the surrounding tissue as being static, based on an assumption that the tissue is not moving. If this is the case, the in-phase and quadrature I and Q data can be filtered separately with simple real filters which remove the DC component and very low frequencies. The cutoff frequency of these high pass filters can be varied for a given application by changing the filter coefficients.
The assumption of static tissue is generally a good one for radiology applications, except in the abdomen, where residual respiratory and cardiac motion cause some amount of tissue motion. In addition, motion of the hand held transducer will also produce an appearance of tissue motion. Since the velocity of this tissue or transducer motion is usually slow compared to the velocity of the blood flow being imaged, the operator can set the wall filter cutoff frequency high enough to filter out the tissue signal component. Filtering in this way, however, will also remove signals from low-velocity blood flow, which are often the signals desired to be imaged.
Some prior art systems provide a wall filter which can be manually adjusted to filter out a narrow band of frequencies in the echo signal centered on the carrier frequency where static signals lie. The bandwidth of this filter must be adjusted so that the signals reflected from the slow moving wall are eliminated without distorting the blood flow measurement. If the filter bandwidth is set too wide, signals from slowly moving blood may be eliminated. In addition, the filter setting is static and is applied over the whole image so that, within the field of view of the image, the filter may work adequately at some locations but not at others.
The processing approach described in Noujaim et al. U.S. Pat. No. 5,349,524 issued Sep. 20, 1994 and assigned to the instant assignee uses adaptive wall filtering whereby the wall signal is mixed down to zero frequency and then removed by using a real time domain filter to filter the I data and the Q data. This reduces the wall signal amplitude and allows the flow signal to be detected with greater accuracy, and at lower velocity than without this method. The adaptive wall filter automatically adjusts its bandwidth as a function of the received echo signal. A complex mixer receives the received echo signal and produces a modified echo signal which is shifted in frequency by an amount equal and opposite to the mean frequency of the received echo signal. The wall filter receives the modified echo signal and filters out a band of frequencies determined by the variance of the received echo signal. By automatically shifting the received echo signal frequency by an amount opposite to its measured mean frequency, the signal components therein resulting from slowly moving tissue are effectively shifted to the center of the filter. By automatically controlling the filter stop band width in dependence on the measured variance, signal components produced by slowly moving tissue are filtered out. The filter output signal is then conventionally processed to produce a color signal indicative of flow velocity.